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1.
This work aimed to study the impacts of acquisition and assimilationof various nitrogen sources, i.e. NO3, NH4+ or NH4NO3,in combination with gaseous NH3 on plant growth and acid-basebalance in higher plants. Plants of C3 Triticum aestivum L.and C4 Zea mays L. grown with shoots in ambient air in hydroponicculture solutions with 2 mol m–3 of nitrogen source asNO3, NH4+ or NH4NO3 for 21 d and 18 d, respectively,had their shoots exposed either to 320 µg m–3 NH3or to ambient air for 7 d. Variations in plant growth (leaves,stubble and roots), and OH and H+ extrusions as wellas the relative increases in nitrogen, carbon and carboxylatewere determined. These data were computed as H+/N, H+/C, (C-A)/N,and (C-A)/C to analyse influences of different nitrogen sourceson acid-base balance in C3 Triticum aestivum and C4 Zea maysplants. Root growth in dry weight gain was significantly reduced bytreatment with 320 µg m–3 NH3 in Triticum aestivumand Zea mays growing with different N-forms, whereas leaf growthwas not significantly affected by NH3. In comparison with C3Triticum aestivum, non-fumigated C4 Zea mays had low ratiosof OH/N in NO3–3-grown plants and of H+/N in NH4+- and NH 4NO3-grown plants. Utilization of NH3 from the atmospherereduced both the OHN ratios in NO3 -grown plantsand the H+/N ratio in NH4+ - and NH4NO3 -grown plants of bothspecies. Furthermore, Zea mays had higher ratios of (C-A)/Nin NH4+ - and NH4NO3-grown plants than Triticum aestivum. Thismeans that C4 Zea mays had synthesized more organic anion perunit increase in organic N than C3 Triticum aestivum plants.Within both species, different nitrogen sources altered theratios of (C-A)/N in the order: NH4NO3>NH4+>NO3.Fumigation with NH3 increased organic acid synthesis in NO3- and NH4+ - grown plants of Triticum aestivum, whereas it decreasedorganic acid synthesis in Zea mays plants under the same conditions.Furthermore, these differences in acid-base regulation betweenC3 Triticum aestivum and C4 Zea mays plants growing with differentnitrogen sources are discussed. Key words: Acid-base balance, ammonia, ammonium, nitrate, ammonium nitrate, C3 Triticum aestivum L., C4 Zea mays L.  相似文献   

2.
The effects of transpiration rate on the vertical gradientsof leaf and stem xylem water potential ( and ) were examinedusing hydroponic sunflower plants. Transpiration was variedby stepwise alterations of environmental conditions. The gradientsof and were relatively small (2.3 and 0.8 x 105 Pa m–1)when transpiration rates approached zero, but increased sharplyto 5.4 and 2.3 x 105 Pa m–1 as transpiration increased.However, the gradients were independent of transpiration ratesabove 0.4 g dm–2 h–1 owing to variability of theplant resistance. The gradients of I were usually less thanhalf those of I. 1 in individual leaves remained constant over a wide range oftranspiration rates (0.4—2.4 g dm–2 h–1) andeach leaf possessed a characteristic plateau value related toits elevation. I responded similarly but was approximately 2.0x 105 Pa higher than I at the same elevation. Identical resultswere obtained regardless of the procedure employed to vary transpiration. The drop in water potential between stem and leaf implies thatthe leaf resistance is appreciable. This was confirmed usingrapidly transpiring excised leaves freely supplied with water.I increased by 2.0–2.5 x 105 Pa following removal of theroot resistance but remained 2 x 105 Pa lower than similar excisedleaves in darkness. Furthermore, I in excised leaves remainedconstant over a wide range of transporting rates, demonstratingthat the leaf resistance is also variable. The results are discussed in relation to previous reports.  相似文献   

3.
The rates of net photosynthesis by closed canopies of tomatoplants were measured at three CO2 concentrations and three humiditiesover a range of natural light flux densities. The data havebeen analysed using a model of canopy photosynthesis which allowsfor variation in leaf area index and other leaf and canopy characteristics.The model also deals explicitly with the effects of CO2 concentration,leaf conductance, and photorespiration on the leaf photochemicalefficiency, . The leaves were found to have a photochemicalefficiency in the absence of photorespiration, m, of 12?6 ?10–9 kg (CO2) J–1. At a CO2 concentration of 0?73 ? 10–3 kg m–3 (400vpm) the leaf photochemical efficiency, , and canopy light utilizationefficiency, c, were 18 per cent greater at a vapour pressuredeficit of 0?5 kPa than at 1?0 kPa. At a CO2 concentration of2?2 ? 10–3 kg m–3 (1200 vpm) they were only 5 percent greater.  相似文献   

4.
Larqué-Saavedra, A., Rodriguez, M. T., Trejo, C. andNava, T. 1985. Abscisic acid accumulation and water relationsof four cultivars of Phaseolus vulgaris L. under drought.—J.exp. Bot 36: 1787–1792. Plants of four cultivars of Phaseolus vulgaris L. differingin drought resistance were grown in pots under greenhouse conditionsand prior to flowering water was withheld from the pots untilthe mid-day transpiration rate reached values below 1.0 µgH2O cm–2 s–1 (designated the ‘drought’stage). At this point leaves were harvested on 3 or 4 occasionsover 24 h to determine the abscisic acid (ABA) concentration,total water potential (), solute potential (1) and turgor potential(p). Results showed that values of , 1, and p differed between cultivarswhen they reached the ‘drought’ stage. The stomatalsensitivity to changes in and p, was as follows: Michoacán12A3 > Negro 150 Cacahuate 72 > Flor de Mayo. These datacorrelated well with the pattern of drought resistance reportedfor the cultivars. ABA accumulation at the ‘drought’ stage differedbetween cultivars at each sampling time, but overall differencesin ABA level between cultivars were not significant. ABA levelsdid not, therefore, correlate with the drought resistance propertiesreported for the cultivars. Results are discussed in relationto and hour of the day when bean samples were taken for ABAanalysis. Key words: Phaseolus vulgaris L., drought resistance, abscisic acid  相似文献   

5.
Smith, J. R. 1987. Potassium transport across the membranesof Chara. II. 42K fluxes and the electrical current as a functionof membrane voltage.—J. exp. Bot. 38: 752–777. The current required to clamp the trans-membrane voltage ofinternodal cells of Chara australis at different levels wasmeasured simultaneously with either the 42K influx or efflux.Examination of the voltage-dependence of the ratio of the electricalcurrent to the unidirectional tracer fluxes yielded no evidenceof any amplification of the electrical driving force on theK+ ions. There was thus no evidence for the interaction of K+ions with themselves or any other species during their passageacross the membrane. These measurements allow the determinationof , the fraction of the electrical current carried by K+ ions.When the external [K+] = 10 mol m–3, the average valueof was 0?85 for Vm > –125 mV and 07?5 for Vm <–150 mV. When the external [K+] = 0?1 mol m–3, was 0?6 for Vm < –80 mV and 0?1 for Vm > –250mV. It was also found that the conductance associated with K+transport was inhibited by hyperpolarization. Key words: Potassium, conductance, flux-ratio  相似文献   

6.
When grown under conditions of low relative humidity, the C3–C4intermediate Panicum milioides, as well as the C3 grasses Triticumaestivum and Poa pratense, exhibited 13C values which were upto 2–7%o less negative than the 13C values of the correspondingplants grown at high relative humidity. At both humidity levels,there was no evidence of a substantial contribution of phosphoenolpyruvatecarboxylase to carbon gain in Panicum milioides  相似文献   

7.
The effects of -hydroxy-2-pyridinemethanesulphonic acid (-HPMS)upon net photosynthesis (Pn, the CO2 compensation point (),post-lower illumination burst of CO2 (PLIB) and post-lower temperatureburst of CO2 (PLTB) in detached rye (Secale cereale L.) leaveswere investigated. At low concentrations ( 0.5 mol m–3),-HPMS initially stimulated Pn and decreased the magnitude ofboth PLIB and PLTB. The decreased at all concentrations of-HPMS (0.05–5.0 mol m–3. The effects of -HPMS onPn and were time-dependent and, after a few minutes, the Pnwas inhibited while values increased considerably. At a higherconcentration (5.0 mol m –3), the transient effects of-HPMS were shorter () or not observed at all (Pn. Both PLIBand PLTB, when expressed in relation to Pn, increased at higherlevels of this compound. Similar data with respect to the effectsof -HPMS on PLIB and PLTB were found for leaves of dandelion(Taraxacum officinale L.). The results suggest that -HPMS may stimulate Pn by inhibitingphotorespiration, as originally suggested by Zelitch (1966),but only at low concentrations and over a short time span. Thedecrease of PLIB and PLTB values at low -HPMS levels is consistentwith these processes being a residual activity of the glycolatepathway. Key words: CO2 compensation point, -hydroxy-2-pyridinemethanesulphonic acid, photorespiration, photosynthesis  相似文献   

8.
It has previously been observed that Clinflux falls withincreasing internal CIconcentration. This paper attemptsto show that this is a bona fide negative feedback involvedin a homeostatic CIaccumulation system. First, predictions are made of how the final steady concentrationto which Clis accumulated ([Cl]1) would be expected tovary as external Clconcentration ([Cl]0) or temperature,change. This is done for the cases where influx has (a) no feedbackcontrol, or is (b) under error-actuated or (c) under reciprocalfeedback control. Secondly, Clinflux and [Cl]1are measured in carrot andmaize root tissue over a range of [Cl]o and temperatures forcomparison with the prediction. [Cl)1varies by only about 20%over a range of [Cl]0 and temperatures which have large immediateeffects on Clinflux. This is consistent only with error-actuatedfeedback control. Changes in [Cl]0 and temperature are bestregarded simply as perturbations of Clinflux since theyhave no marked effect on [Cl)1. In such a homeostatic system the feedback signal is a functionof (CR – [Cl], ), where CR represents the value of aninbuilt set point. CR must be constant with [Cl]o and temperature,and its nature is briefly discussed. It is further predictedthat the accumulated level can only be altered in a controlledmanner by a change in CR, and that this would alter influx bythe same proportion. The effects of abscisic acid in increasingboth [Cl]1 and Clinflux are consistent with an effectprimarily on CR. These conclusions imply that when varieties or species differin levels accumulated, they do so not because influx isothermsdiffer but rather because their set points differ. Key words: Homeostasis, Chloride, Accumulation  相似文献   

9.
The components of leaf water potential (l) and relative watercontent (RWC) were measured for stands of bambara groundnut(Vigna subterranea) exposed to three soil moisture regimes incontrolled-environment glasshouses at the Tropical Crops ResearchUnit, Sutton Bonington Campus. Treatments ranged from fullyirrigated (wet) to no irrigation from 35 days after sowing (DAS)(dry). RWC values varied between 92–96% for the wet treatment,but declined from 93% to 83% in the dry treatment as the seasonprogressed. l at midday decreased in both the wet and dry treatments,but the seasonal decline was more pronounced in the latter:seasonal minimum values were –1.19 and –2.08 MPa,respectively. Plants in the wet treatment maintained turgor(p) at about 0.5 MPa throughout the season, whereas values inthe dry treatment approached zero towards the end of the season.There was a linear relationship between p and l9 with p approachingzero at a l of –2.0 MPa. Mean daily leaf conductance wasconsistently higher in the wet treatment (0.46–0.79 cm-1)than in the intermediate and dry treatments (0.13–0.48cm s-1 Conductances in the intermediate and dry treatments weresimilar, and the lower evapotranspirational water losses inthe latter were attributable to its consistently lower leafarea indices (L): L at final harvest was 3.3, 3.3 and 1.9 forthe wet, intermediate and dry treatments. Bambara groundnutwas apparently able to maintain turgor through a combinationof osmotic adjustment, reductions in leaf area index and effectivestomatal regulation of water loss. Key words: Vigna subterranea, water relations, soil moisture  相似文献   

10.
The effect of Chromium VI on leaf water potential (w), solutepotential (a), turgor potential (p) and relative water content(RWC) of primary and first trifoliatc leaves of Phaseolus vulgarisL. was studied under normal growth conditions and during anartificially induced water stress period in order to establishthe possible influence of this heavy metal on the water stressresistance of plants. Plants were grown on perlite with nutrientsolution containing 0, 1•0, 2•5, 5•0 or 10•0µg cm–3 Cr as Na2Cr2O7.2H2O. The effect of Cr onwater relations was highly concentration dependent, and primaryand first trifoliate leaves were affected differently. The growthreducing concentrations of Cr (2•5, 5•0 and 10•0µg cm–3) generally decreased s and w and increasedp in primary leaves. The 1•0 µg cm–3 Cr treatmentdid not affect growth, but altered water relations substantially:in primary leaves w and p were increased and s decreased, whilein trifoliate leaves the effect was the opposite. All Cr treatedplants resisted water stress for longer than control plants.The higher water stress resistance may be due to the lower sand to the increased cell wall elasticity observed in Cr VItreated plants. Key words: Phaseolus vulgaris, Chromium VI, water stress, Richter plot  相似文献   

11.
In Trifolium repens L. there were immediate transient depolarizationsof the membrane electropotential (Evo) when KH2PO4 was addedto phosphate-free media, but these were of the same magnitudeas the controls (K2SO4 and KCI). Furthermore, the extents ofdepolarization were the same as the expected effect of the addedK+ calculated using the Goldman equation. There was no significantdepolarization on adding H3PO4 to buffered media. Consequently,there was no evidence for a depolarization caused by phosphate.This result provides evidence that the H+–H2PO4 symportin roots of T. repens operates with a stoichiometry of 1: 1. In a group of control plants ( + P plants) and a group whichwere stressed by reducing the supply of phosphate (– Pplants), the – P plants had lower values for Evo than+P plants (– 118 mV and – 130 mV, respectively).The absence of phosphate from the measurement media also reducedEvo (mean effect = 9 mV). A significant difference in Evo between– P and + P plants persisted when phosphate was addedto – P plants. The electropotential difference acrossthe tonoplast (Evo) in – P plants became more positivewith time. Key words: White clover, membrane transport, roots, tonoplast, symport  相似文献   

12.
By analysing the relationship between inverse water potential(–1), and relative water content (RWC) measured on leavesof roses (Rosa hybrida cv. Sonia), grown soilless, it was foundthat a non-linear (NL) model was better suited than a linearmodel to reproduce values observed in the non-turgid region.To explain this apparent curvature, it is assumed that a reductionof the non-osmotic water fraction (Ap) takes place when decreases.Osmotic potentials () measured on fresh and frozen leaf discstend to support this hypothesis. A method for exploiting PVcurves, which takes into account the variation of Ap, is described.It delivers values for the turgor pressure (p), the relativeosmotic water content, and the mean bulk volumetric elasticitycoefficient, lower than those given by the linear model. Onthe other hand, it gives higher estimates for Ap and for . Whenapplying the traditional model to obtain estimates for waterrelations characteristics of rose leaves, and comparing resultsfrom two distinct salinity treatments (electrical conductivitiesof 1·8 mS cm–1 and 3·8 mS cm–1, respectively),one deduces a significant reduction of at turgor-loss in thehigh salinity treatment. The NL method is, in addition, ablesimultaneously to reveal a reduction of and a significant increasein p at RWC=100% this proves that soilless–grown roseplants are able to osmoregulate when subjected to a constantand relatively high degree of salinity. Key words: Apoplastic water, non-linear regression, pressure-volume curves, tissue-water relations  相似文献   

13.
The vacuolar pH (pHv) and the cytoplasmic pH (pHc) of the marinegiant-celled green alga Chaetomorpha darwinii were measuredby pH microelectrode techniques on extracted vacuolar sap, andby the [I4C]DMO distribution method respectively. Equilibrationof DMO occurred with a half-time of about 2 h, with an apparentPDMO of 3.6 x 10–5 cm s–1, but the vacuolar concentrationof free, undissociated DMO was always less than the externalconcentration. The explanation offered for freshwater giant-celledalgae of net DMO leakage across the plasmalemma cannotapply to Chaetomorpha darwinii, since electrically-driven DMOexit from the cytoplasm should be similar across the plasmalemmaand the tonoplast in these cells with large, vacuole-positivepotential differences across the tonoplast. pHc was accordinglycomputed assuming either tonoplast or plasmalemma equilibrationof DMO, with correction for DMO metabolism within the cell.pHc was 8.0–8.3 in the light in artificial seawater (pHoabout 8.0), was some 0.5 units lower in the dark, and was slightlylower with an external pH of 7. Vacuolar pH was 6.5–6.9,without consistent effects of illumination or of external pHof 7 rather than 8. While µH+ at the tonoplast was similarto that in giant-celled freshwater algae (although with a greatercontribution from relative to pH), µH+ at the plasmalemmawas less than 8 kJ mol–1, i.e. less than one-third ofthe value in freshwater green algae. µNa+ was some 13kJ mol–1 at the plasmalemma. The possibility that theprimary active transport process at the plasmalemma of Chaetomorphadarwinii (and certain other marine algae) is Na+ efflux ratherthan H+ efflux is discussed.  相似文献   

14.
Thomas, H. 1987. Physiological responses to drought of Loliumperenne L.: Measurement of, and genetic variation in, waterpotential, solute potential, elasticity and cell hydration.—J.exp. Bot. 38: 115–125. Clonally-replicated genotypes of Loiium perenne L. were grownin a controlled environment. Leaf water potential (w) osmoticpotential (s), turgor potential (p = ws), elasticity(E), leaf hydration (g water per g dry matter, H) and numberof green leaves per tiller (NGL) were measured before and duringa 42 d drought treatment. A simplified method of estimating E (at w < 1?0 MPa) usingonly six measurements was developed to permit a measurementrate of 8 leaves per hour. Measurement errors in all characterswere 3% or less. During drought, w and s (at w = 0?5 MPa) decreased significantly,p and E increased significantly, and H decreased slightly. Plantsize during drought was negatively correlated with s, and Hand positively correlated with p, osmotic adjustment, E andNGL. Measurements made on the genotypes before draughting didnot give a reliable indication of their physiological conditionafter adaptation to drought. Genetically controlled variation (‘broad sense heritability’)of drought-adapted plants for E was 15%, w 23%, s, 34%, p, 35%,H 34% and NGL 64%. The possibilities for, and effectivenessof, divergent selection of genotypes with high and low expressionof the characters are discussed. Key words: Water relations, Lolium, genetic variation  相似文献   

15.
The dynamic and steady state aspects of the pH and electricpotential () profiles, that develop in the experimental mediumin association with the photosynthetic assimilation of exogenousHCO3 by internodal cells of Chara corallina, were investigated.A theoretical treatment is presented which explains the originof the phenomenon. This theory was tested by comparing thepH and values generated by a numerical analysis model (whichsimulated the experimental system) against experimental data.Verification of our model indicates that the steady state ionicfluxes, associated with HCO3 assimilation (HCO3,OH, and CO23, are not significantly influencedby the electric potential gradients. The main driving forcecausing the observed fluxes is the diffusion gradient associatedwith the respective ion. By simultaneous measurement of and pH, at the centre of analkaline band, a direct correlation was established betweenlight-activation and dark-deactivation of the OH transportsystem and the light-mediated changes in at the cell surface.In addition, under steady state conditions, an almost perfectcorrelation was observed between alkaline band pH centres andthe negative electric potential maxima. These data offer strongsupport for the hypothesis that OHefflux, in this system,is an electrogenic process. Based on our present analysis, the profile along the cell indicatesthat, in terms of the spatial aspect of HCO3 transport,the rate of HCO3 influx varies quite dramatically alongthe length of an internodal cell. This aspect is discussed interms of the cellular integration of OH and HCO3transport in this species.  相似文献   

16.
The effect of high light and root chilling on gas exchange,chlorophyll fluorescence, and bulk shoot water potential (shoot)was examined for Pinus sylvestris seedlings. Transferring plantsfrom low light (200 µmol m–2s–1, PAR) anda soil temperature of 15 °C to high light (850 µmolm–2 s–1) and 1 °C caused >90% decrease innet photosynthesis and leaf conductance measured at 350 mm3dm-3 CO2, and a decrease in the ratio of variable to maximumfluorescence (Fv/Fm) from 0.83 to 0.63. The decrease in Fv/Fmwas, however, only marginally greater than when seedlings weretransferred from low to high light but kept at a soil temperatureof 15 °C. Thus, photoinhibition was a minor component ofthe substantial decrease observed for net photosynthesis at1 °C soil temperature. The decrease in net photosynthesisand shoot at 1 °C was associated with an increase in calculatedintracellular CO2 concentration, suggesting that non-stomatalfactors related to water stress were involved in inhibitingcarbon assimilation. Measurements at saturating external CO2concentration, however, indicate that stomatal closure was thedominant factor limiting net photosynthesis at low soil temperature.This interpretation was confirmed with additional experimentsusing Pinus taeda and Picea engelmannii seedlings. Decreasesin gas-exchange variables at 5 °C soil temperature werenot associated with changes in shoot Thus, hormonal factors,localized decreases in needles or changes in xylem flux maymediate the response to moderate root chilling.  相似文献   

17.
Henson, I. E. 1985. Solute accumulation and growth in plantsof pearl millet (Pennisetum americanum [L.] Leeke) exposed toabscisic acid or water stress.—J. exp. Bot. 36: 1889–1899.Experiments were conducted to investigate whether abscisic acid(ABA) elicits the accumulation of solutes and lowering of osmotic(solute) potential (2) which occurs in leaves of pearl millet(Pennisetum americanum [L.] Leeke) exposed to water stress.When (?)–ABA was injected into the base of the shoot of15–d–old plants, 2 of the fifth leaf was reducedsignificantly below controls 27–72 h after treatment.The reductions, however, were small (< 0.10 MPa) and wereaccompanied by a significant inhibition of shoot growth. Incontrast, no significant reduction in 2 or in growth occurredwhen the hormone was introduced directly into leaf five viaits mid–rib. ABA concentrations in leaf five were highshortly after direct injection, but declined to control levelswithin 48 h. Injecting ABA into the shoot base resulted in lowerleaf five ABA concentrations. Hence, the ABA concentration inthe leaf was not the most critical factor for its effect on2. ABA also reduced 2 of shoots when applied to seedlings 48h or 72 h after sowing via the roots. As with older plants,the effects of ABA on 2 were small ( 0–2 MPa) and wereaccompanied by inhibition of shoot growth. A water stress treatmentand an ABA treatment were compared. Although both treatmentsresulted in a similar degree of growth inhibition, the stresstreatment was much more effective than was ABA in reducing 2. Key words: Pennisetum americanum [L.], pearl millet, abscisic acid, water stress, osmotic  相似文献   

18.
Corrigendum     
Light response curves for (•) gross 16O2 evolution, and() CO2 uptake in 210 mmol mol–1 O2 with 900–1000µbar CO2 or () in air by leaves of Hirschfeldia incana.The difference between (•) and () or () was quantitativelyequivalent to the measured 18O2 uptake. The areas under thecurves are labelled to identify regions of assimilatory andnon-assimilatory electron flow redrawn from data of Canvin etal. (1980). It should be noted that the data and the labelling of the figureaxes are correct as printed.  相似文献   

19.
The natural abundance of 13C in different parts of beech (Fagussylvatica L.) leaves was analysed. Values for leaf ribs wereconsistently higher than those for intercostal tissue. Similardifferences occur between petiole and stem, with petiole beingless negative. The pattern of results is the same, independentof the position in the tree. However, the absolute values differby up to 6%. from the bottom to the top of the tree. Valuesof 13C are in the range of – 29 to – 32%. for thelower leaf strata; while values between – 24 and –26%. have been measured for the top of the tree. Absolute 13Cvalues of the whole tissue and cellulose differ by about 2%.,but relative 13C trends are almost identical. However, 13C trendsare not identical for different leaf parts. A comparison ofcellulose and whole tissue 13C data makes it unlikely that the13C variations are primarily due to different compositions ofchemical compounds. No fractionation seems to exist betweenleaf and wood cellulose. Tissue from different areas of a leafrevealed identical carbon isotope compositions. Key words: Carbon isotope ratio, Fagus sylvatica L., beech leaves  相似文献   

20.
The daily cycle of the transpiration rate, stomatal conductance,water potential, and the concentration of the main osmoticumidentified in ash leaves, malate and mannitol, were monitoredin a field on the Isere river plain. On sunny days, the stomatalconductance tends to remain close to its maximun value allowinga high transpiration rate and diurnal variations in leaf waterpotential, w, which may fall as low as –2 MPa at solarnoon. These variations of w are closely correlated with changesin malate, mannitol and the concentration of the well-knownosmoticum K+, which agree with the involvement of an osmoticadjustment to counteract the evaporative demand during daylighthours. How malate, mannitol and K+ contribute to the osmoticadjustment was analysed subsequently by comparing the solutepotential s, evaluated by the Boyle-Van't Hoff relation, tothe osmotic potential measured by thermocouple psychrometry.These experiments have led us to suspect some errors in themeasurement of , presumably due to experimental artefacts andthe ability of Ca 2+ , present in high levels in leaves, toform chelates with malate once the cells have been decompartmentedby freezing and thawing. Since significant changes of Ca2+ alsooccurred during the diurnal variations of w, the possible mechanismsby which Ca2+ may be implicated in controlling the water statusof the tree are discussed. Key words: Fraxinus excelsior L, osmotic adjustment, thermocouple psychrometry, malate, calcium  相似文献   

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